This invention relates to image pick-up tube targets and, more particularly, to an image pick-up tube target capable of effectively suppressing sticking of images.
As is known in the art, amorphous selenium (Se) has photoconductivity and when it is combined with a signal electrode of "N" conductivity type a photodiode type photoconductive target is provided. Since Se is not sensitive to long wavelength radiation, it has been proposed to add Te into a portion of the Se-containing layer in order to improve the sensitivity to long wavelength radiation. This method is disclosed in U.S. Pat. No. 3,890,525 to Hirai et al. and U.S. Pat. No. 4,040,985 To Shidara et al.
FIG. 1 shows a basic structure of a typical target of the prior art, such as that disclosed in the U.S. Pat. No. 4,040,985. In FIG. 1, numeral 1 designates a transparent substrate, numeral 2 designates a transparent N-type conductive layer, and numeral 3 designates the first P-type photoconductive layer which corresponds to a sensitized portion of a P-type photoconductive film. Numeral 4 denotes the second P-type conductive layer which serves to reduce an electrostatic capacitance of the target, and numeral 5 denotes a beam landing layer for assisting landing of a scanning electron beam.
The first P-type photoconductive layer 3 may be made of Se, As and Te and the region where the signal current is generated by conversion of light energy into electrical current for the most part and FIG. 2 shows an example of concentration distribution in the direction of thickness from the transparent N-type conductive layer side of the first P-type photoconductive layer 3 shown in FIG. 1. In the illustrated example, sensitizing Te does not exist at zero level of the film thickness (region a) corresponding to the interface with the transparent conductive layer 2, and the concentration of Te rapidly increases from a 100 nm level of the film thickness and Te is added over the thickness of 150 nm (region b). The element As is added in the regions a and b to enhance the thermal stability of Se. Region c is added with As which is considered to form deep levels in the energy gap to enhance the effect of sensitization. The concentration of As decreases at a uniform rate over the film thickness of 250 nm. This As also serves to enhance the thermal stability of Se. The target which has the structure of this type attains an object for increasing sensitivity to long wavelength radiation.
The second P-type photoconductive layer 4 may be made of Se and As which is added to enhance the thermal stability of Se, is thick enough to minimize electrostatic capacitance which causes capacitive lag, and constitutes most of the thickness of the photoconductive target.
The beam landing layer 5 may be made of Sb.sub.2 S.sub.3, and forms a porous layer by being evaporated in low vacuum while the first and second P-type photoconductive layers 3 and 4 usually form glassy layers by being evaporated in high vacuum.
The image pickup tube of this type exhibits good characteristics to the ordinary requirements for image pick-up tubes such as lag and after-image. A part from the above, it has been proposed to dope a small amount of halogen in order to improve the lag and the afterimage of the target used in a pickup tube whose main component is Se and which utilizes rectifying contact. This method is disclosed in U.S. Pat. No. 3,984,722 to Maruyama et al. The target of this type exhibits good characteristics in ordinary usage condition, but if the intensity of incident light becomes considerably higher than during a normal usage condition, the response after the incident light has been cut off (lag for intense light) is deteriorated. The longer the operating time becomes, the more deteriorated this response is. The normal usage condition used herein is determined as the condition in which the intensity of the incident light is such that it produces a signal current output of approximately 0.2 .mu.Ap-p. The intensity of the light used herein is roughly 20 times as high as that of the normal usage condition, although the value is not defined critically. The lag resulting from such an intense light is usually referred to as "high light sticking." In order to minimize the high light sticking, it has been proposed to dope at least one kind of metal fluoride selected from a group consisting of LiF, NaF, MgF.sub.2, CaF.sub.2, BaF.sub.2, AlF.sub.3, CrF.sub.3, MnF.sub.2, CoF.sub.2, PbF.sub.2, CeF.sub.3 and TlF into the region where the signal current is generated for the most part. This method is disclosed in U.S. Pat. No. 4,330,733 to Shidara et al.
However, when a stationary scene is viewed continuously for a long time with an image pick-up tube having the above-mentioned target, a so-called sticking phenomenon occurs wherein a previously viewed image sticks and superposes on a presently viewed image. Physical mechanism of this sticking may be explained as follows. Photogenerated carriers produced in illuminated areas of the target are captured in carrier traps of the photoconductive layer, form space charges, modify internal electric field across the layer, and modulate sensitivity of the target locally while in non-illuminated areas of the target, an electric field across the photoconductive layer is uniform over the entire areas and determined by voltage applied from an external power source. The difference between the sensitivity of previously illuminated areas and that of previously non-illuminated areas results in sticking.
Japanese Unexamined Patent Publication (Kokai) No. SHO 57 (1982)-80637 corresponding to U.S. Pat. No. 4,563,611 discloses a method in which fluoride is doped over a region where the signal current is generated for the most part for the purpose of promoting recombination of carriers.
However, in the target constructed in accordance with the above-mentioned patent, in a region where Te is doped in order to enhance sensitivity to long wavelength radiation, space charges due to trapping of photogenerated carriers modify the internal electric field across the photoconductive layer locally and decrease an amount of photocurrent to be drawn therefrom. This shifts photocurrent-voltage characteristic curves toward higher voltage and causes difference between sensitivity of previously illuminated areas and that of previously non-illuminated areas and results in sticking.